Various types of respiration and heartbeat monitors are known in the art for monitoring cyclic movement within the thorax of an individual, such as a post-operative patient or an infant subject to sudden crib death, for the purpose of actuating an alarm in case of trouble. Among these is one that is disclosed in the copending U.S. patent application Ser. No. 07/190,177, filed May 4, 1988 by Fred Sterzer, and assigned to the same assignee as the present invention. As disclosed in this patent application, one module is situated adjacent the chest and another module is situated adjacent the back of a post-operative patient. A carrier signal, which may have a frequency of 40 Mhz, is transmitted from a first of these modules through the thorax of a patient to second of these modules. At the second module, the carrier signal is modulated by a given modulation frequency and then retransmitted back to the first module. At the first module, the retransmitted signal is homodyned with the carrier frequency in a frequency converter that provides a filtered output that rejects the frequency-converted baseband, but passes the frequency-converter modulation frequency. Both the frequency-converted baseband and modulation frequency will be doppler-shifted in accordance with cardiopulmonary movements within the thorax. However, spurious doppler frequency shifts due to back-scatter of the originally transmitted carrier signal from moving objects not within the thorax of the patient (i.e. carrier-signal leakage that has never been modulated by the modulation frequency) occurs only in the rejected frequency-converted basdband. Such a non-invasive monitor minimizes the undesired occurrence of false alarms, while providing a high signal-to-noise cyclic cardiopulmonary waveform for analysis, thereby permitting accurate determination of whether or not an abnormal condition exists that requires the actuation of an alarm.